Malaria parasites seduce mosquitoes on the sly. Plasmodium falciparum parasites produce a molecule that makes parasite-infected blood more attractive to malaria-transmitting mosquitoes, researchers report online February 9 in Science. The insects slurp up this enticing meal, helping the parasite spread to new hosts. “It is a really intriguing glimpse into how Plasmodium might have evolved to enhance its probability of transmission,” says Conor McMeniman, a mosquito researcher at Johns Hopkins University who wasn’t part of the study.

Previous research has suggested that mosquitoes might be preferentially drawn to malaria-infected people, but it was unclear what piqued their interest. Biologist Noushin Emami of Stockholm University and colleagues got an unexpected lead when studying the effect of a molecule called HMBPP on the immune system of Anopheles gambiae mosquitoes, major spreaders of malaria. P. falciparum releases this molecule into the bloodstream of its hosts. While watching mosquitoes sip blood from artificial feeders, the researchers “noticed that the mosquitoes ate a lot more from the blood in this artificial feeding system when the HMBPP was in the blood,” says biologist Ingrid Faye, also at Stockholm University. “That was the thing that made us think [the molecule] changes [the mosquito’s] behaviour.”

HMBPP, or (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate, didn’t directly lure the mosquitoes. When it was mixed with serum, which doesn’t contain red blood cells, the mosquitoes weren’t as interested. But red blood cells with added HMBPP released more carbon dioxide than regular red blood cells, and also produced greater amounts of certain airborne chemicals called aldehydes and monoterpenes. That aroma attracted more mosquitoes, and those insects ate larger than usual meals.

Mosquitoes sense the carbondioxide (CO₂) that humans exhale and use it as a cue to find food. So it makes sense that increased CO₂ would draw more mosquitoes. Aldehydes and monoterpenes — compounds also released by plants — might attract mosquitoes by making humans smell a bit like the plants mosquitoes get nectar from, says McMeniman. But since the molecule’s allure was tested in artificial mosquito feeders, it’s still unclear how strongly the molecule lures mosquitoes to infected humans, he says.